The present invention relates to a negative electrode for non-aqueous secondary battery and a non-aqueous secondary battery.
As non-aqueous secondary batteries, there have been put to practical use lithium ion secondary batteries which use a non-aqueous electrolyte and use lithium ion for charge and discharge reaction. The lithium ion secondary batteries are higher in energy density as compared with nickel metal hydride batteries and the like, and are used as secondary batteries for power sources of portable electronic equipments. However, the lithium ion secondary batteries as power sources are required to be higher in capacity and smaller in size with the recent portable electronic equipments becoming higher in capacity and smaller in size. For meeting the requirement, it is essential to increase the capacity of negative electrodes.
At present, carbon materials are used for negative electrode active materials used in negative electrodes, and lithium ion is occluded/released by intercalation/deintercalation of lithium ion between grapheme layers, and the theoretical capacity thereof is 372 Ah/kg. However, remarkable increase of capacity cannot be expected because actual capacity close to theoretical capacity has been realized for carbon materials. Therefore, substitutes for carbon materials are vigorously searched and high capacity can be expected. A high interest centers around alloy negative electrodes (or metal negative electrodes) which carry out charge and discharge reaction by alloying/dealloying reaction as shown in the formula of xLi++M+xe− LixM (M indicates a metal). For example, silicon has a theoretical capacity of 4200 Ah/kg and tin has a theoretical capacity of 990 Ah/kg which are several times—10 times the theoretical capacity of carbon materials.
However, it has been known that the alloy negative electrodes are greater than carbon materials in volume change caused by charge and discharge reaction, and silicon expands to 420% and tin expands to 360% at the time of alloying of lithium ion, and the structure of battery container cannot be maintained due to the stress caused by the great volume change with charging and discharging, resulting in cycle characteristics inferior to those of carbon materials and thus improvement in this respect is required.
Under the circumstances, the following Patent Document 1 proposes that the structure of battery container is maintained and cycle characteristics are improved by alloying with a matrix component which does not react with lithium ion, but the cycle characteristics are still inferior and the negative electrodes could not be put to practical use.
In the following Patent Documents 2-8, it is proposed to use a material containing an imide compound as a binder. Patent Document 2 proposes that adhesion between a negative electrode mix layer and a collector is improved by using a material containing an imide compound as a binder of negative electrode, and that pores are formed in the negative electrode mix layer for preparing a binder by decomposing a binder precursor by heat treatment, thereby to relax the stress caused by expansion and contraction of negative electrode active material particles. However, since the negative electrode active material per se has no pores, the structure of the negative electrode active material per se is broken to cause deterioration of cycle characteristics.
Further, the following Patent Document 9 proposes a method of decreasing the volume change by a porous body having pores comprising a continuous solid produced by plating a foamed metal with a metal element alloying with lithium ion to inhibit collapse of the structure, but according to this method, the pore diameter is large, resulting in low strength that is not preferred.
The following Patent Document 10 proposes to improve cycle characteristics by forming voids in a porous particle comprising only silicon to prevent apparent changes in volume. However, since the negative electrode comprises only silicon and contains no matrix component, relaxation of stress caused by charging and discharging is insufficient, and besides low conductivity is brought about, that is not preferred.    Patent Document 1: JP-A-2009-032644    Patent Document 2: JP-A-2007-242405    Patent Document 3: JP-A-2009-252580    Patent Document 4: JP-A-2009-164104    Patent Document 5: JP-A-2008-016446    Patent Document 6: JP-A-2007-165061    Patent Document 7: JP-A-2005-197096    Patent Document 8: JP-A-2001-185152    Patent Document 9: JP-A-2004-022512    Patent Document 10: JP-A-2004-214054